The present invention relates to a vertical suspension smelting process and device which makes it possible to process finely-divided copper- and/or nickel-bearing concentrates into metal and reject matte. The object of the invention is to provide a process by which especially sulfidic and/or oxidic concentrates rich in iron can be processed advantageously in terms of both technology and economy.
In recent years the object in developing copper processes has been to perform the various stages of the process, i.e., smelting, conversion and slag-purification processes, in one and the same unit, whereby a lessening of the environmental hazards caused by the sulfur-bearing reaction gases is achieved in addition to certain economic and technological advantages.
When refining ferriferous concentrates, the common problem in all processes is the great atmospheric sensitivity of the slag phases produced in the process. The use of gases with a high oxygen potential, necessary in the oxidation processes, causes a sharp increase in the ferric iron and copper contents of the slag phases. In such a case a sufficient lowering of the valuable-metal contents of the slag phases respectively require an effective reduction process. The processes developed for manufacturing copper differ from each other mainly in the manner in which the various stages of the process are combined. The known basic processes, which have been continuously developed, can be classified into flash furnace, direct conversion, and suspension processes. It is mainly suspension processes that are discussed in this connection. In the process according to U.S. Pat. No. 3,687,656, a horizontal cyclone oxidation system (known from, for example: L. M. Rafalovich, V. L. Russo: Tsvetnye Metally, No. 9, 1964, pp. 30-39) has been linked to a chamber which has been provided with one or more partitions at times reaching the melt in the furnace tank and which operates according to the principle of communicating vessels. When the matte surface in the surface tank has risen sufficiently owing to the processing, the slag phase separated into the chamber following the first chamber is reduced by iron sulfide spraying, whereby the obtained valuable-metal matte settles in the tank. The obtained reject slag is removed from the system and the exposed sulfide matte is blasted into metal in the same or (depending on the iron content of the matte) the following chamber (surface blast). After the discharge of the raw metal the matte-slag phase accumulated in the first chamber during the partial processes spreads into the furnace tank below the partitions, and the process continues in a manner corresponding to the initial situation. The discharge of the reaction gases from the system takes place through the upper part of the chambers.
In the process according to U.S. Pat. No. 3,555,164, a vertical cyclone oxidation system is used. It has been linked to a smelting chamber provided with a dam wall against the cyclone (in horizontal systems, the wall opposite the dam wall is against the cyclone: Rafelovich and U.S. Pat. No. 3,687,656). One end of the smelting chamber has been linked to an electric heating furnace by means of a gas partition extending into the melt. The reaction gases are fed into the purification process from the other end of the smelting chamber. Between the smelting chamber and the electric heating furnace and on the side of the electric heating furnace there is an accumulation container for separating the matte from the slag. The metal vapors obtained in the sulfur-free atmosphere in the electric heating furnace during the course of the slag reduction are condensed into raw metal. Judging from the patent specification it is possible to produce copper, according to the process, directly from, for example, chalcocite concentrates. However, the system might be suitable for ferriferous copper and nickel concentrates only when these valuable metals are recovered in the form of sulfide matte. In such a case condensing raw metal (Zn, Cd, Hg, etc.), sulfide matte and waste slag are obtained from the electric heating furnace. As examples of complete suspension processes, in which the discharge of the suspension mainly occurs against the melt surface of the furnace part (in cyclone smelting, already in the cyclone device), we can mention the horizontal oxidation processes producing low-grade sulfide mattes and, when provided with a periodic iron sulfide spraying also waste slags (e.g., U.S. Pat. No. 2,668,107), and the vertical suspension systems which produce high-grade mattes and metals. The first vertical suspension process implemented on a technological scale is the one according to U.S. Pat. No. 2,506,557. The process is used to produce sulfide mattes poor in valuable metals and those rich in valuable metals, as well as raw metals. The continuous contact between the sulfide matte and the slag phase according to the process prerequires, even in equilibrium, relatively high valuable-metal concentrations in the slag phase, and therefore, according to the patent specification, a reverberatory or an electric furnace has been linked to the system for the purpose of slag reduction. It should be mentioned that, according to the original process, metallic copper and slag which requires only a slight reduction (a function of the Fe, Co, and Ni contents of the concentrate) can be produced directly on an industrial scale from, for example, chalcocite concentrates. The basic process has been developed to a great extent during the past couple of decades. In another method, a process for sulfidizing the slag and the flying dust in the lower furnace and the rising shaft has been developed for the reduction, into an equilibrium, of the slag phase obtained in producing high-grade sulfide mattes, in which case, by regulating the degree of reduction of the gas phase, either a partial or complete separation of elemental sulfur form the gas phase is achieved. In the process according to U.S. Pat. No. 3,754,891 a post-oxidation reduction process for the sulfide concentrate in the reaction shaft has been developed. The process makes it possible to produce iron-poor nickel matte from nickel-poor, highly ferriferous sulfide concentrates. The process is based on a selective resulfidization, in the furnace tank, of most of the nickel oxide produced by oxidizing the concentrate, for example, before it combines into olivine in the slag phase. Two new processes should also be mentioned among the applications of the vertical suspension smelting process.
In the process according to U.S. Pat. No. 3,674,463, the object has obviously been to improve the flash smelting process (U.S. Pat. No. 2,506,557) in order to make it suitable for iron-rich sulfide concentrates. The process is discussed in more detail than is usual because of both its general nature and the close links between it and the flash smelting processes. A conventional flash smelting furnace is used in the process. The oxidation of the suspension is also performed in a conventional flash smelting reaction shaft. The ratio between oxygen and the feed sulfides has been regulated so that 35-75% of the copper present in the feed mixture is converted into metal. Deviating from the conventional flash smelting process, copper concentrate is fed into the lower part of the reaction shaft. The purpose is to produce a very low oxygen pressure in the lower part of the reaction shaft, that is, less than 10.sup.-5 mm Hg (1.3.times.0.0.sup.-8 atm) close to the slag surface. According to the patent specification, the sprayed additional concentrate serves as a protection against a high partial pressure of oxygen, which would otherwise cause magnetite formation. In the process, both the sulfide conversion and the control of the ferric iron in the product of oxidation are thus performed in suspension.
When evaluating the new process on the basis of the theory and practice of flash smelting, it can be noted that with the conventional height of the reaction shaft, temperature of the suspension, and flow rates of the flash smelting system, a direct conversion into metal of the sulfide in suspension is not possible. It is true that a small part of the copper is always recovered in a metallic form from the reaction shaft, but this metal is produced as a result of the reactions of the shaft product impining against the shaft wall. Most of the metal phase in the flash smelting system is not produced until in the lower-furnace reactions (e.g., partial oxidation of chalcocite: 2 Cu.sub.2 O(l)+Cu.sub.2 S(l).revreaction.6Cu(l)+SO.sub.2 (g). In the production of high-grade copper mattes the metal phase at the bottom of the furnace tank is produced when the superoxidized shaft product metallizes copper sulfide in the furnace tank, and the metal phase segregates into a separate phase owing to the sulfide-metal-melt solubility gap. Most of the metallic copper present in the solid slags of high-grade sulfide mattes originates in the reactions taking place in the slag at the solidification stage (e.g., the decomposition reaction of the remanent melt and wustite: 3FeO(s)+Cu.sub.2 O(s).revreaction.Fe.sub.3 O.sub.4 (s)+Cu(s)).
A few observations on the reactions in the suspension according to the description of the process of the above patent, with reference to FIG. 3: The stability ranges of the systems Cu-S-O and Fe-S-O as functions of the partial pressures of sulfur and oxygen in the atmosphere, at 1200.degree. C., calculated on the basis of known thermodynamic functions, are shown in FIG. 3A. The description of the process gives values P.sub.O.sbsb.2 =1.32.times.10.sup.-8 -1.32.times.10.sup.-10 atm as the limits of the oxygen pressure in the gas phase after the spraying of concentrate. These pressure limits are indicated by indices Y-1 and Y-2 in FIG. 3-A. Respectively, the gas phase compositions corresponding to the oxygen pressures have been calculated from the (incomplete) material balance in the description. They are indicated in the drawing by indices Y-11 and Y-12 (i.e. P.sub.SO.sbsb.2 =0.17 and 0.04 atm) According to the diagram, each composition is approached mainly in the direction of arrows I and II. In case I, copper oxide (possibly metallic) and copper sulfide are stable in the oxidation path when the oxygen pressure lowers. Magnetite produced primarily under a high oxygen pressure (air oxidation) is stable all the time. Thus, the suspension product obtained in an equilibrium is a mixture of magnetite and molten copper sulfide. In case II, the course of the oxidation reactions is analogous, but the final equilibrium (Y-22) prerequires the presence of the product mixture molten copper sulfide and iron sulfide in the suspension. It should be noted in this connection that the oxygen pressure of the system cannot be reduced alone, e.g., along a low sulfur isobar from the range of solid magnetite to the wustite range, since the sulfur dioxide present in the gas phase is known to be reduced extremely rapidly and at the same time the partial pressure of the sulfur of the gas phase increases respectively. Thereby, magnetite is both reduced and sulfidized. In an atmosphere completely devoid of sulfur, the lowering of the oxygen pressure in the suspension is known to lead to a magnetic reduction rate determined by a very slow diffusion of solids, which is not of the same order as the sulfidizing rate in a gas phase containing elemental sulfur. A rapid sulfidization is due to the effect of the molten phase (Fe-S-O) produced on the magnetite surface and detaching from it, in which case the reduction and the sulfidization occur on an almost pure oxide surface (the product of sulfidization "departs").
On the basis of the flash smelting theory and practice, it is not easy in the process according to the patent description to prevent a rapid sulfidization of the copper or copper oxide in the suspension in the gas atmosphere prerequiring a stable sulfide salt, as set forth in the description. It should also be mentioned that the conversion, according to the description, of a high-grade sulfide matte in suspension in a separate zone (shaft) is very difficult. A partial oxidation of the sulfide matte in suspension and an oxide)sulfide conversion taking place in the furnace tank on the other hand, do produce results (known from, for example, U.S. Pat. No. 2,209,331).
The recent development of continuous-working suspension systems producing metal from concentrate is illustrated by the system described in U.S. Pat. No 3,460,817. In this system, the oxidation of iron-free copper matte from concentrate is performed in a conventional flash-smelting shaft. The copper rate and the slag phase are separated in the furnace tank. The copper matte accumulates in the space below the shaft and flows from there, under the partition extending into the matte phase, into the conversion part of the furnace system (Arutz siphon). In the conversion space the iron-free sulfide matte is blasted into metal by means of oxygen (air) by either surface or tuyere blasting. Opposite the conversion part in relation to the reaction shaft there is the slag-treatment zone (reverberatory). The slag reduction is performed with molten iron sulfide, the produced poorgrade copper matte flowing against the slag coming from under the reaction shaft. Molten iron sulfide is produced by oxidizing pyritic or other sulfide concentrates in suspension with a limited air quantity in a second reaction shaft close to the slag discharge end of the furnace system (known from U.S. Pat. No. 3,306,708). In the system according to the description, great attention has thus been accorded to the treatment of the slag phase.
In FIG. 3-A there is a curve A which indicates the position, in the stability field, of the iron sulfide mattes obtained in the suspension smelting of pyrite (on an industrial scale). Under conventional industrial conditions the operation takes place with low gas-phase oxygen pressures P.sub.O.sbsb.2 =10.sup.-9 -10.sup.-10 atm as the atmosphere consists essentially of sulfur. In the operation pyrite is only smelted, not oxidized and thus the oxygen partial pressure remaining low. According to the diagram, the iron sulfide activity in the mattes is very low (a.sub.FeS =0.4-0.6) owing to the effect of the dissolved oxygen. For low copper concentrations in the waste slags, large quantities of reducing sulfide must be used in the reduction (matte-slag separation). This results in a considerable increase in the waste slag quantity. It must be noted, however, that the system according to the patent description can obviously be used for producing metallic copper continuously.
The technological level of reverberatories and conversion furnaces producing metal directly from concentrate will also be discussed. The recent development of reverberatory processes is well illustrated by the Worcra smelting process and its applications, of which U.S. Pat. Nos. 3,326,671 and 3,527,449 can be mentioned as examples. In the process according to U.S. Pat. No. 3,527,449, which uses an improved reverberatory, metal (Cu) is produced in one unit directly from concentrate. The process includes smelting, conversion and slag-removing zones, the first one being situated in the center of the furnace. The conversion is performed with air pipes. The lowering of the sulfur concentration in the obtained product to a value corresponding to raw metal can be performed by converting it with an Arutz siphon in a furnace tank part separate from the matte and slag phases. The slag is purified by a pyrite wash in the slag-removal zone.
The processes according to Canadian Pat. No. 758,020 and Pat. Application No. 104,111 can be mentioned as examples of the present level of continuous conversion processes. A modified Peirce-Smith-type converter is used for the conversion. The blasting of the concentrate into raw metal is performed in one or more zones. In addition to raw metal, a slag phase rich in valuable metals is obtained from the system (4.5-12% Cu), and this slag phase is treated outside the converter by known processes (reverberatory, froth flotation, etc.).